Building monitoring and control systems including security systems, HVAC and other monitoring and control systems are in increasing use in both commercial buildings and residential dwellings. For security systems, the increasing use is due in part to a long-term perception of increasing crime rates along with increasing awareness of the availability of building monitoring and security systems. For HVAC systems, the increasing use is due in part to the desire to reduce heating and cooling costs, and to save energy.
A building monitoring and/or control system typically includes a variety of remote units coupled to detection devices and at least one master unit which typically resides in a central location in the building and can include annunciation functions and reporting functions to another location such as a central reporting service or police department. Remote units have, in the past, been hard wired to the master unit. For example, in a security system, reed switches or Hall effect switches are often disposed near magnets located near doors and door jambs, with a door opening making or disrupting continuity, with the resulting signal being received by the master unit.
In hardwired systems the remote units and the detection devices may be nearly one in the same. For example, the detection device may be a foil trace on a glass pane and the remote unit may be wire terminals with optional signal conditioning equipment leading to a wire pair connected to the master unit. Hard wired units can be installed most easily in new construction, where running wire pairs is easier than in existing buildings. Installing hard-wired systems can be very expensive in existing buildings due in part to the labor costs of snaking wires through existing walls and ceilings. In particular, on a point-by-point basis, retrofitting residential dwellings can be expensive because houses are often not designed to be continually changed, as are many office buildings. For example, most houses do not have dropped ceilings and utility closets at regular intervals. Houses can have higher aesthetic expectations than commercial office buildings, requiring greater care in installing and concealing wiring.
Wireless security systems have become increasingly common. Existing systems use radio frequency transmission, often in the 400 MHz region. Wireless systems can greatly reduce the need for wiring between remote and master unit or units. In particular, wireless systems can communicate between the remote units and the master units without wiring. Remote units still require power to operate, and can require wiring to supply that power, which can add a requirement for power wiring where the power had been provided in hard wired systems over the wiring used to communicate between remote units and the master unit. The power requirement can partially negate the wireless advantage of radio frequency units, as some wiring is still required. The power supply wiring requirement is often eliminated with use of batteries. Battery life is largely a function of power consumption of the remote units. The power consumption is dependent upon both the electronics and upon the transmission duty cycle of the unit.
Current wireless systems typically utilize remote units that can only transmit and master units that can only receive. Remote units often transmit sensor data for needlessly long periods, and at higher power than is required, as there is no bi-directional capability, and therefore no way for the master unit to acknowledge receipt of the first remote unit message, or a low power message. Sometimes, the remote units transmit a health status message at regular periodic intervals. The health status message gives the health of the remote unit, sometimes includes sensor data, and informs the master unit that the remote unit is still functioning.
The periodic transmissions can be scheduled at the remote units by manually setting DIP switches or providing local programming to the remote units. However, the scheduling of the remote unit transmissions typically cannot be controlled or adjusted by the master unit because the communication between master and remote units is unidirectional. The master simply has no way to notify and change the timing of transmissions provided by the remote units. Since there is no coordination between the transmission times of the remote units, collisions can occur between remote unit transmissions, which may reduce the overall reliability of the system. To increase the probability that a particular remote unit transmission is received by the master, the remote unit may make the same transmission many times. However, this can significantly increase the power consumed by the remote units.
What would be desirable, therefore, is a bi-directional wireless monitoring and/or control system that has predetermined or periodic remote unit transmissions coordinated on a system wide basis so as to reduce or avoid collision between the scheduled transmissions. This may significantly increase the reliability of the system, and may reduce the power consumed by the remote units.